When his rivals have stopped battering each other long enough to criticize him, they've often done so tentatively and ham-handedly. Romney's injury-free journey is all the more surprising because, despite some obvious campaign skills, he has well-known vulnerabilities ripe for attack.

The turn of events has astonished campaign pros in both parties, who expected Romney to be more bloodied. And it has dismayed President Barack Obama's allies, who assumed Republicans would at least soften up the man they viewed as the likeliest nominee from the start.

"By all rights, Romney should have spent the last six months with a target painted on his back," said Dan Schnur, a former GOP adviser who teaches politics at the University of Southern California. "But he has been able to keep his head low," Schnur said, while a series of rivals have taken turns quarreling, surging and falling.

New polls show Romney heading into Tuesday's caucus as the front-runner in a state that seems ill-suited to his background, and which snubbed him four years ago. The Iowa Republican caucus is usually dominated by evangelical voters, home-schoolers and other social conservatives. Yet his rivals have done little here to turn those dynamics against Romney, a Mormon who supported legalized abortion and mandatory health insurance as governor of liberal Massachusetts.

Romney began this year's campaign de-emphasizing Iowa. But his rivals' inability to produce a clear leader has opened a possible path for him to seize the prize.

A Romney win in Iowa, which is far from certain, would make him the clear favorite to win the nomination. Next up is the Jan. 10 primary in New Hampshire. Romney has a second home there, and the GOP voters' greater emphasis on financial matters is better suited to his politics.

Romney's luck stems largely from his opponents' early conclusion that he had enough money and experience to go deep into the nominating contest, and only one viable alternative could emerge. They've been competing for that spot, and attacking each other, ever since.

"If you have modest resources, you're going to spend your time differentiating yourself from the rest of the non-Romney crowd," said GOP lobbyist and strategist Mike McKenna.

Campaign attack ads in Iowa underscore the point. When former House Speaker Newt Gingrich surged in polls earlier this month, he was quickly pilloried by TV ads and mailings financed by groups associated with Romney, Texas Gov. Rick Perry and Rep. Ron Paul of Texas.

In two weeks in Iowa, a PAC that supports Romney dumped $2.6 million into the effort, according to records compiled by the Center for Responsive Politics. Having little money to respond, Gingrich has plummeted in the polls.

A far smaller sum was spent on anti-Romney ads, mostly by a pro-Obama group trying to fill the vacuum.

Campaign veterans say Perry had the best chance to establish himself early as the Romney alternative. That could have positioned him to hammer away at his Massachusetts rival. A proven fundraiser with 10 years as Texas governor, Perry rocketed to the top of GOP polls when he announced his candidacy in mid-August.

But he quickly fell after debate performances that typified the GOP field's inability, or unwillingness, to train sustained fire on Romney's obvious soft spots: his changed positions on abortion, gun control and gay rights, and his policy of requiring Massachusetts residents to buy health insurance or pay a penalty.

In a Sept. 22 debate in Florida, Perry started to describe Romney's various flip-flops. But he stammered and wandered so badly that it was nearly impossible to understand his point.

After that, the GOP field showed little interest in launching focused attacks on Romney's policy changes.

Romney was equally lucky in June. Then-candidate Tim Pawlenty had said in a TV interview that "Romneycare" was the inspiration for "Obamacare," the GOP term for the Democrats' 2010 health care overhaul. But in a televised debate that followed, Pawlenty refused to repeat the criticism. His reticence contributed to his fast decline, and gave Romney a big break.

Conservative columnist George Will said on ABC in September: "Tim Pawlenty got in trouble when he got a chance to attack Romney and didn't. Perry's in trouble because he attacked Romney and did it so incompetently."

Romney has enjoyed other breaks. James Pethokoukis of the conservative American Enterprise Institute noted that Romney recently stopped short of endorsing a value-added tax without ruling it out in all circumstances.

"Many conservatives/libertarians simply hate, hate, hate the idea of a VAT," Pethokoukis wrote. "I would be surprised if those quotes don't end up in a 30-second, anti-Romney ad in Iowa or New Hampshire."

Rodell Mollineau heads American Bridge, a Democratic group that gathers information to use against Republicans in campaigns. This year's GOP candidates have tried to attack Romney at times, Mollineau said, "but they're just not very good at it."

"Perry has the money, but he can't get a sentence out," Mollineau said. Gingrich is articulate, he said, but hasn't raised enough money to wound Romney with broadcast ads.

Romney's luck continued Wednesday. Rep. Michele Bachmann criticized two rivals during her fast-moving bus tour of Iowa. She said Perry has spent "27 years as a political insider," and Paul would be "dangerous as president" because of his hands-off views on national security.

Romney largely escaped her ire.

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